What are the visa stay dates for a Non-Imm O-A visa holder in Thailand and when should I apply for an extension?

@Jean-Francois *********

you are confusing TWO completely different things.

You hold a valid Non-O-A VISA (the sticker in your passport that you got from the thai consulate in your country).

HOWEVER

It's the tiny red entry stamp that lets you stay here. You are allowed to stay until Dec 3, 2021 on it

You have two options;

Exit re-enter the country on or before the date the VISA (sticker) expires (Nov 19 2021) and get a new year entry stamp (if you have covid and health insurance for another year

OR

Go to the immigration office and apply for a yearly extension of stay before the 3rd of Dec by showing you have 800K baht in the bank account for 2 months before you apply for the extension AND by having the 40k/400k mandatory health insurance by one of the 15 approved OA insurance carriers here in thailand.

Your entry stamp allows you to stay till Dec 3. You can apply for your one year extension 30 days before then. Your visa sticker which gets you into the country, expires Nov 19, if the borders were open, you could do a border bounce before then and receive a stamp for another year, since it is a multi o-a, but since that isn't available, it will simply expire.

The 90 day report date is not connected to your visa/extension expiry date. You just need to make another 90 day report before 19 November. Your permission to stay expires on 3 December. You can extend it 30 days or at some offices 45 days in advance. Your extension will run from 3 December, so no disadvantage for applying early.

Is this your first year of this visa ??

Go and talk with imigration much easyer